Question

Assertion :Micelles are formed by surfactant molecules above the critical micellar concentration (CMC). Reason: The conductivity of a solution having surfactant molecules decreases sharply at the CMC.

• A. The Assertion is true, Reason is true; Reason is a correct explanation for Assertion
• B. The Assertion is true, Reason is true; Reason is not a correct explanation for Assertion
• C. The Assertion is true, Reason is false
• D. The Assertion is false, Reason is true

Answer 1

The correct option is B The Assertion is true, Reason is true; Reason is not a correct explanation for Assertion

There are some substances which at low concentrations behave as normal strong electrolytes but at higher concentrations exhibit colloidal behaviour due to the formation of aggregated particles. The aggregated particles thus formed are called micelles. These are also known as associated colloids.

The formation of micelles takes places only above a particular temperature called Kraft temperature $\left(T_k\right)$ and above a particular concentration called critical micelle concentration ($CMC$).
Each micelle contains at least $100$ molecules. Therefore, conductivity of the solution decreases sharply at the $CMC$ as number of ions decreases due to aggregation.